Device packages with low stress assembly process

ABSTRACT

A microelectromechanical device package and a low-stress inducing method for packaging a microelectromechanical device are disclosed in this invention. The microelectromechanical device is accommodated within a cavity comprised by a first package substrate and a second substrate, wherein a third substrate is disposed between and bonded to both the microelectromechanical device lower semiconductor substrate and the package bottom substrate. The first and second package substrates are then bonded so as to package the microelectromechanical device inside.

CROSS-REFERENCE TO RELATED CASES

This application is a divisional application of co-pending U.S. patentapplication Ser. No. 10/698,656 to Tarn, filed Oct. 30, 2003, thesubject matter being incorporated herein by reference in entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is generally related to the art of packagingdevices, and more particularly, to packaging stress sensitive devices.

BACKGROUND OF THE INVENTION

Stress sensitive devices, such as optical devices (e.g. photo detectors,CCDs, LCD, photodiodes), microelectromechanical systems (e.g. spatiallight modulators using micromirrors) may suffer from device failure dueto warping and stresses induced either during device packaging processesor in operation after improper packaging processes. In a typicalpackaging process, the device is attached to a package substrate forholding the device. When the coefficient of thermal expansion (CTE) ofthe package substrate does not match the device substrate that contactsthe package substrate, the device will be warped, resulting in devicefailure.

An approach to solve this problem is to select device substrate andpackage substrate having the same or similar CTEs. However, this is notachievable in many situations because the selection of the devicesubstrate and also the package substrate need to satisfy otherrequirements with higher priority.

Therefore, a method is desired for packaging stress sensitive deviceswith a low stress assembly process, while allows for employing a widerange of adhesives and packaging materials.

SUMMARY OF THE INVENTION

In view of the foregoing, the present invention discloses a method ofpackaging stress sensitive devices. Stresses, such as thermal stressesand warpage are reduced to tolerable levels by providing one or moresubstrates between the device and package substrates.

According to an embodiment of the invention, a substrate of a packagefor packaging a micromirror array device is provided therein. Thesubstrate comprises: According to another embodiment of the invention, amethod of packaging a micromirror array device is disclosed. The methodcomprises: providing a first package substrate; attaching asemiconductor device or a microelectromechanical device to anintermediate substrate layer; attaching said contraption to the firstpackage substrate; placing a second substrate on the first packagesubstrate; and bonding the first and second substrate using appropriatetechniques.

BRIEF DESCRIPTION OF THE DRAWINGS

While the appended claims set forth the features of the presentinvention with particularity, the invention, together with its objectsand advantages, may be best understood from the following detaileddescription taken in conjunction with the accompanying drawings ofwhich:

FIG. 1 a is a diagram schematically illustrating a packaging substratefor packaging a micromirror array device, the packaging substrate havinga heater that is formed along the periphery of one surface of thesubstrate and embedded underneath said surface of said substrateaccording to an embodiment of the invention;

FIG. 1 b is a cross-sectional view of the package substrate of FIG. 1;

FIG. 2 is a diagram schematically illustrating a packaging substratehaving a heater that is laminated between two layers of said packagingsubstrate according to another embodiment of the invention;

FIG. 3 a is a diagram schematically illustrating a micromirror arraydevice that is packaged using a packaging substrate in FIG. 1 accordingto another embodiment of the invention;

FIG. 3 b is a cross-sectional view of the package in FIG. 3 a;

FIG. 4 a is a diagram schematically illustrating a micromirror arraydevice that is packaged using a packaging substrate in FIG. 1 accordingto yet another embodiment of the invention;

FIG. 4 b is a cross-sectional view of the micromirror array package ofFIG. 4 a;

FIG. 5 a is a diagram schematically illustrating a micromirror arraydevice that is packaged using a packaging substrate of FIG. 2 accordingto yet another embodiment of the invention;

FIG. 5 b is a cross-sectional view of the micromirror array device ofFIG. 5 a;

FIG. 6 a is a cross-sectional view of the micromirror array device ofFIG. 3, schematically illustrating the adhesive layers used for bondingthe micromirror array device to an intermediate substrate and to thepackage according to one embodiment of the invention;

FIG. 6 b illustrates a cross-sectional view of the micromirror arraydevice of FIG. 3, depicting the different adhesive layers used forbonding the micromirror array device to an intermediate substrate and tothe package according to another embodiment of the invention;

FIG. 6 c is a cross-sectional view of the micromirror array device ofFIG. 3, schematically illustrating the different exemplary adhesivelayers and substances used for bonding the micromirror array device toan intermediate substrate and to the package according to yet anotherembodiment of the invention; and

FIG. 6 d illustrates a cross-sectional view of the micromirror arraydevice of FIG. 3, depicting the different exemplary adhesive layers andsubstances used for bonding the micromirror array device to anintermediate discontinuous substrate and to the package according to oneembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings, the present invention is illustrated as beingimplemented in a suitable packaging process for stress sensitivedevices, such as optical devices (e.g. photo detectors, CCDs, LCD,photodiodes) and microelectromechanical systems (e.g. spatial lightmodulators using micromirrors). The following description is based onselected embodiments of the invention and should not be interpreted as alimitation of the invention with regard to alternative embodiments thatare not explicitly described herein.

Referring to FIG. 1 a, a packaging substrate for packaging stresssensitive devices is illustrated therein. Packaging substrate 200comprises substrate layer 210 and substrate layer 215. Substrate layer210 has a concave surface that forms a cavity in which the stresssensitive device can be disposed. As an alternative feature, heater 220is formed along the periphery of the concave surface of substrate layer210. Electric current from external electric power source can beintroduced into heater 220 via two leads 222 so as to generating heat.Heater 220 is laminated between substrate layers 210 and 215. Across-sectional view of packaging substrate 200 is illustrated in FIG. 1b.

Substrate layers 210 and 215 can be any suitable preferablynon-electrically conducting materials, preferably ceramic or glass, andmore preferably ceramic. Other materials (e.g. organic or hybridorganic-inorganic materials) could also be used depending upon theirmelting points. In another embodiment of the invention, substrate layers210 and 215 each can be a multilayered structure that further comprisesa plurality of substrate layers. In this situation, the top layer, onwhich the heater is disposed, of substrate 210 and the bottom layer,which face the heater, of substrate 215 are preferably non-electricallyconducting. Other layers, including the substrate layers underneath thetop layer of substrate 210 and the substrate layers above the bottomlayer of substrate 215 can be any desired materials, such as ceramic,glass and metallic materials.

As discussed above, substrate layer 210 has a concave surface that formsa cavity in which micromirror array device can be placed. Alternatively,the substrate layers can be flat plates, as shown in FIG. 2. Referringto FIG. 2, substrate layers 266 and 262 of package substrate 260 bothare flat plates. Alternatively, heater 220 is provided. The heater isformed on substrate layer 266 and along the periphery of the surface ofsubstrate layer 266 and is laminated between substrate layers 266 and262. Stress sensitive device, such as micromirror array device 105 canbe bonded to substrate layer 106 and supported thereby by packagesubstrate 262. Alternatively, the intermediate substrate layer 106 canbe attached to package substrate 262 and further on provide support formicromirror array device 105. Substrate 106 consists preferably of amaterial with similar mechanical properties to that of the lowersubstrate material of the micromirror array device 105 (e.g. Si).

In the following, exemplary implementations of the embodiments of thepresent invention will be discussed with reference to packages ofmicromirror array devices and packaging processes for making the same.It will be understood by those skilled in the art that the followingexemplary implementations are for demonstration purposes only and shouldnot be interpreted by any ways as a limitation. In particular, althoughnot limited thereto, the present invention is particularly useful forpackaging semiconductor devices or micromirror array devices. Themethods and packages disclosed can also be applied in packaging othertype of stress sensitive devices, such as MEMS-based optical switches,image sensors or detectors and semiconductor devices. Other variationsof the packaging substrates without departure from the spirit of thepresent invention may also be applicable. For example, the packagingsubstrate layers can be any desired shapes, other than the preferredrectangular shape.

Referring to FIG. 3 a, a micromirror array device package using thepackaging substrate with an integral heater in FIG. 1 is illustratedtherein. Specifically, micromirror array device 105 with attachedsubstrate layer 106 is packed in the cavity of packaging substrate 200,which comprises integral heater 220 as shown in FIG. 1 and theintermediate layer 106 attached to said package substrate. A doublesubstrate type micromirror device is illustrated, however, as in all thedrawings, a single substrate device (e.g. micromirrors formed on siliconwafer) could be used. Cover substrate 235, which is preferably glass, isprovided for sealing the micromirror array device within the cavity. Inorder to bond cover substrate 270 and packaging substrate 200, sealingmedium 230, preferably one that forms a hermetic seal and has a meltingtemperature of 300° C. or less, and preferably 200° C. or less, isdisposed between the cover substrate and packaging substrate as shown.Preferably the sealing material is an inorganic material such as ametal, metal alloy or metal compounds (e.g. a metal or metalloid oxide).Alternatively, sealing medium layer 230 can also be deposited directlyon the surface of packaging substrate 200, or on the surface of thelower surface of cover substrate 235, in which case, sealing mediumlayer 230 is preferably deposited along the periphery of the lowersurface of the cover substrate. Sealing medium 230 is preferably amaterial that is stable, reliable, cost-effective and has goodthermal-properties (e.g. co-efficient of thermal expansion (CTE),thermal-conductivity etc.) compatible with the other components, such aspackage substrate 200 and cover substrate 235, of the micromirror arraydevice package. It is further preferred that sealing medium has a lowmelting temperature (when the sealing medium is non-metallic) orsoldering temperature (when the sealing medium is metallic). Glass frit,such as Kyocera KC-700, is an acceptable candidate for the sealingmedium. During the bonding process, an electric current is driventhrough the integral heater via the two heater leads (i.e. leads 222)for generating heat. The amplitude of the electric voltage is dominatedby electric characteristics of the heater (e.g. electric properties ofthe material of the heater, the shape of the heater), thermalcharacteristics and geometry of the substrate layers of packagingsubstrate 200 and the desired temperature on the surface of packagingsubstrate 200 for melting sealing medium (e.g. sealing medium layer230). As an example, the melting temperature, also the desiredtemperature on the surface of packaging substrate 200, of sealing medium230 is from 100 to 300° C., preferably around 350° C. The heater isembedded underneath the surface of the packaging substrate at a distancepreferably from 1 millimeter to 10 millimeters, preferably around 7millimeters. In this example, the packaging substrate is ceramic. Thenthe voltage set up between the two heater leads 222 is preferably from40 to 100 volts, preferably around 70 volts. In other words, thisvoltage causes the heater generating heat with an amount that raises thesurface temperature of the packaging substrate to the meltingtemperature of sealing medium layer 230. As a result, sealing medium ismelted and used to bond cover substrate 235 and packaging substrate 200.Meanwhile, the temperature at the micromirror device location is farless than the temperature that causes mechanical failure of themicromirrors of the micromirror device. In the embodiment of theinvention, the temperature at the micromirror device location ispreferably less than 70° C.

During the bonding process, external pressure may be applied to thecover substrate, as shown in FIG. 3 b, wherein a cross-sectional view ofFIG. 3 a is illustrated therein. After a predetermined time period whenthe cover substrate and the packaging substrate are securely bonded, thevoltage, as well as the external pressure, can be withdrawn, but notnecessarily at the same time. As shown in FIG. 3 b, one or more getters325 can be provided within the package 270 for absorbing moistures andimpurity particles (e.g. organic particles) either sealed within thecavity or emitted from the components of package 270 during thepackaging process, especially during the heating process.

Though cover substrate 235 is preferably visible light transparentglass, it may also consist of other materials, such as metals ormaterials that are not transparent to visible light. In these cases,cover substrate 235 preferably comprises an inlay light transparentglass for allowing light to travel through and shine on micromirrorarray device 105. Alternatively, cover substrate 235 may have an openingforming window with a light transparent glass mounted on the window forallowing transmission of incident light. Moreover, a light blocking maskwith light blocking strips formed around the circumference of the maskmay be applied along cover substrate 235 for blocking incident light notshining on the surface of the micromirror array device. By this, opticalperformance, such as contrast ratio, of the micromirror array device canbe improved.

Other than using glass frit as sealing medium, other suitable materials,such as solderable metallic materials, such as Au, BiSn_(x), AuSn_(x),InAg_(x), PbSn_(x), and copper, may also be used. However, mostsolderable metallic materials have poor adhesion to oxide materials orlayers that often form on surfaces of the substrates. To solve thisproblem, a metallization film is preferably employed to metalizing thesurface of the substrate before using solderable metallic sealingmediums, which will be discussed in further detail in the following.

Referring to FIG. 4 a, sealing medium layer 245 comprises a solderablemetallic material that is preferably stable, reliable, cost-effectiveand has thermal-properties (e.g. co-efficient of thermal expansion(CTE), thermal-conductivity etc.) compatible with the other components,such as package substrate 200 and cover substrate 235, of themicromirror array device package, and more preferably has a lowsoldering temperature. In order to enhancing adhesion of sealing mediumlayer 245. to the surfaces of substrates 235 and 200, metallizationlayers 240 and 250 are provided for metalizing the lower surface ofcover substrate 235 and top surface of package substrate 200,respectively. Metallization mediums can be any suitable materials, suchas aluminum, gold, nickel or composition of two or more of suitablemetallic elements, such as gold/nickel, preferably a material with lowsoldering temperature. These materials can be deposited on the surfacesas thick or thin films using suitable deposition methods, such as thosestandard methods (e.g. sputtering) for depositing thin film and thosestandard methods (e.g. print and paste) for depositing thick films. Inan embodiment of the invention, metallization medium layer 250 is a thinlayer of noble metallic material, such as gold. This metallizationmedium layer is preferably deposited, such as sputtered as a film on thelower surface of cover substrate 235. Similarly, another metallizationlayer 240 is provided between sealing medium layer 245 and packagesubstrate 200 for metalizing the top surface of the package substrate.Metallization layer 240 is also preferably deposited, such as sputteredas a film on the upper surface of package substrate 200. Whenmetallization layers 250 and 240 are respectively deposited on the lowersurface of cover substrate 235 and the upper surface of substrate 200,these metallization layers may have high soldering temperatures. In thissituation, theses metallization layers are integral with cover substrate235 and substrate 200, respectively. Alternatively, metallization layers250 and 240, each could be a multilayered structure. As an example, themultilayered structure comprises a metal-oxide layer (e.g. CrO₂ andTiO₂), a metallic layer (e.g. Cr and Ti), a second metallic layer (e.g.Ni) and a third metallic layer (e.g. Au) on top. The metal-oxide layeris first deposited on the surface of the non-metallic substrate, such asceramic and glass, because it presents strong adhesion to thenon-metallic substrate's surface, which is generally oxidized. Themetallic layer generally comprises a metallic material that has strongadhesion to the metallic-oxidation layer. The second metallic layer isdeposited between the third metallic layer and the first metallic layerto prevent diffusion of the first metallic material into the thirdmetallic layer on top. As another example, metallization layer 240further comprises a tungsten layer, a nickel layer and a gold layer. Ofcourse, metallization medium layer 250 may also be a multilayeredstructure that further comprises a plurality of metallization layers asdesired. The third metallic layer on top, preferably comprises ametallic material having low oxidation. Exemplary metallic materials forthe third metallic layer are, Au, Cr and other noble metals.

During the packaging process, the integral heater embedded underneaththe surface of package substrate 200 is electrically powered forgenerating heat so as to solder sealing medium layer 245 betweenmetallization layers 240 and 250. Meanwhile, external pressure may beapplied to the package for enforcing bonding package substrate 200 andcover substrate 235, as shown in FIG. 4 b.

In another embodiment of the invention, cover substrate 235 may alsohave a heater. As the heater (e.g. heater 220) in package substrate 200as described with reference of FIG. 1 a, the heater in cover substrate235 can be formed along the periphery of the surface of the coversubstrate and embedded underneath said surface of the cover substrate.This heater in the cover substrate can be used in bonding the coversubstrate and the package substrate. And it is especially useful insoldering metallization medium layer 250 and sealing medium layer 245.

A cross-sectional view of package 275 in FIG. 4 a is illustrated in FIG.4 b. As seen, other features, such as getters 325 can be provided forabsorbing moisture.

Referring to FIG. 5 a, a micromirror array device package using thepackage substrate as shown in FIG. 2 according to further embodiment ofthe invention is illustrated therein. As seen, package substrate 300 isa flat plate with integral heater 220 embedded underneath the surface ofthe package substrate. Micromirror array device 105 is bonded tosubstrate layer 106 and supported thereby by package substrate. Spacer310 is placed on the package substrate and forms a space along withpackage substrate 300 and layer 106 for accommodating the micromirrorarray device. Cover substrate 320 is placed above the spacer and thepackage substrate. In order to bonding the package substrate, the spacerand the cover substrate into a micromirror array device package, sealingmedium layers 315 and 305 are provided between the cover substrate andthe spacer, and between the spacer and the package substrate,respectively. In the embodiment of the invention, package substrate 300and spacer 310 are ceramic. Alternatively, spacer 310 can be, Kovar,Invar, and NiFe_(x). And cover substrate 320 is light transparent glass.Sealing medium layers 315 and 305 are glass frit. During the packagingprocess, heater 220 is electrically powered for generating heat so as tomelt sealing medium layers 305 and 315. Alternatively, external pressure(not shown) can be applied to enforcing the bonding.

As discussed above, cover substrate 320 is glass for allowing incidentlight traveling through to shine on the micromirror array device.Alternatively, the cover substrate can be a ceramic or metallic materialor any other desired materials that are not transparent to visiblelight. In this case, the cover substrate comprises a window with inlayglass for allowing incident light passing through. Alternatively, aglass plate may be mounted on the window of the substrate that is notincident light transparent. As a further alternative feature of theembodiment, a light blocking mask (e.g. a rectangular frame) that blocksincident light around the periphery of the micromirror array device isattached to the surface of the cover substrate, or directly painted orotherwise deposited around the circumference of the cover substrate.This is particularly useful when the cover substrate is glass.

Other than the flat shape, the cover substrate can be a concave covercap (not shown) with the lower surface of the cover substrate extendedtowards the opposite surface (e.g. the top surface) of the coversubstrate. In this case, the cover cap and package substrate 300 canform a space for housing the micromirror array device without spacer310. Accordingly, the number of metallization medium layers and thenumber of sealing medium layers can be reduced, and bonding process canbe simplified. For example, when cover substrate 320 that is a cover capand package substrate 300 is provided for housing micromirror arraydevice 105, packaging processes described with reference to FIGS. 3 aand 4 a can be directly applied herein.

Referring to FIG. 5 b, a cross-sectional view of the micromirror arraypackage in FIG. 5 a is illustrated therein. In addition to the packagingsubstrate, the intermediate layer attached to it, the cover substrate,the sealing medium layer and the metallization medium layers, gettersmay also be formed within the package. The cover light transmissivesubstrate need not be parallel to the lower substrate, and themicromirror array device, such as set forth in U.S. patent applicationSer. No. 10/343,307, filed on Jan. 29, 2003 to Huibers, the subjectmatter of which being incorporated herein by reference.

Referring to FIG. 6 a, a cross-sectional view of a micromirror arraydevice package using the packaging substrate with an integral heater ofFIG. 1 is illustrated therein. Specifically, a double substrate typemicromirror device 105 is shown with two separate adhesive layersbonding said substrates. An even adhesive layer further bond said devicewith substrate layer 106 and the same method is employed in bondinglayer 106 with package substrate 210. The addition of substrate layer106 to the bonding process of the micromirror device to the packagingsubstrate allows for a wide range of adhesives and packaging materialsto be adapted into the assembly process, since most of the stressinduced during assembly and packaging is now supported by insert 106instead of micromirror device 105. However, substrate layer 106 cansuffer from flexure and/or creep, which can be transferred tomicromirror device 105, due to this induced stress and the CTE mismatchof the materials. In order to prevent this, FIG. 6 b shows a differentpattern in adhesive dispensing for bonding the double substratemicromirror device 105 to layer 106 according to another embodiment ofthe invention.

Substrate layer 106 is in this case attached to micromirror device 105using small quantities of adhesive substance, which successfullyminimizes flexure problems but may create instead a heat transferproblem between the two substrates due to the reduction of contactsurface. As a further alternative, FIG. 6 c illustrates anotherdispensing pattern for the adhesive layer that bonds substrate layer 106to the double substrate micromirror device 105. Another alternative tothis problem is shown in FIG. 6 d, which illustrates a cross-sectionalview of a micromirror array device package using the packaging substratewith an integral heater of FIG. 1 according to yet another embodiment ofthe invention. In this embodiment the two substrates of micromirrordevice 105 are bonded together using a doped adhesive substance, whilesubstrate insert 106 is now a discontinuous layer bonded with adhesivesubstance to both micromirror device 105 and package substrate 210.Alternatively, multiple discontinuous substrate inserts can existbetween the assembly and the package, which greatly reduces flexureresulted from the CTE mismatch and/or deformation of the bondingmaterials or the insert itself.

It will be appreciated by those skilled in the art that a new and usefulmicromirror array package and methods of applying the same for packagingmicromirror array devices have been described herein. In view of themany possible embodiments to which the principles of this invention maybe applied, however, it should be recognized that the embodimentsdescribed herein with respect to the drawing figures are meant to beillustrative only and should not be taken as limiting the scope ofinvention. For example, those of skill in the art will recognize thatthe illustrated embodiments can be modified in arrangement and detailwithout departing from the spirit of the invention. In particular, otherprotective materials, such as inert gas, may be filled in the spaceformed by the package substrate and the cover substrate. For anotherexample, the package substrate, as well as the cover substrate and thespacer, can be other suitable materials, such as silicon dioxide,silicon carbide, silicon nitride, and glass ceramic. For yet anotherexample, other suitable auxiliary methods and components, such asapplications of Infrared Radiation during bonding for soldering thesealing medium layers, and pillars or other structures for aligning thesubstrates are also applicable. Moreover, other desired materials, suchas anti-stiction material, preferably in vapor phase for reducingstiction of the micromirrors of the micromirror array device, may alsobe deposited inside the package. The anti-stiction material can bedeposited before bonding the cover substrate and lower substrate. Whenthe cover substrate (e.g. cover substrate 235 in FIGS. 3 a and 3 b) isglass that is visible light transmissive, it can be placed parallel tothe micromirror array device (e.g. device 105 in FIGS. 3 a and 3 b) andthe package substrate. Alternatively, the cover substrate may be placedat an angle with the micromirror array device or the package substrate.Therefore, the invention as described herein contemplates all suchembodiments as may come within the scope of the following claims andequivalents thereof.

1. A packaged microelectromechanical device, comprising: amicroelectromechanical array device that comprises a semiconductorsubstrate; a package for the microelectromechanical array device, thepackage comprising a ceramic package substrate; a discontinuous insertsubstrate that is disposed between the semiconductor substrate and theceramic package substrate; and. wherein the discontinuous insertsubstrate has a CTE value that is the same as a CTE value of thesemiconductor substrate or between the value of the semiconductorsubstrate and a CTE value of the package substrate.
 2. The device ofclaim 1, wherein the semiconductor substrate is silicon.
 3. The deviceof claim 2, wherein the microelectromechanical array comprises a lighttransmissive substrate bonded to the semiconductor substrate.
 4. Thedevice of claim 3, wherein the light transmissive substrate is glass orquartz.
 5. The device of claim 3, wherein the microelectromechanicalarray comprises a plurality of micromirrors formed on the lighttransmissive substrate.
 6. The device of claim 5, wherein at least500,000 micromirrors are disposed on the light transmissive substrate.7. The device of claim 1, wherein the microelectromechanical array areformed directly on the semiconductor substrate.
 8. The device of claim1, wherein the package substrate is ceramic.
 9. A packagedmicroelectromechanical device, comprising: a microelectromechanicalarray device that comprises a semiconductor substrate; a packagesubstrate having a cavity in which the microelectromechanical arraydevice is disposed; and a discontinuous insert substrate that isdisposed between the semiconductor substrate and the package substrate.10. The device of claim 9, wherein the package substrate is ceramic. 11.A packaged microelectromechanical device, comprising: amicroelectromechanical array device that comprises a semiconductorsubstrate; a ceramic package substrate having a supporting surface; anddiscontinuous insert substrate that is disposed between thesemiconductor substrate and the supporting surface of the ceramicpackage substrate.
 12. The device of claim 11, wherein the supportingsurface is within a cavity of the ceramic package substrate.